Direct chip attachment (DCA) refers to a semiconductor assembly technology wherein an integrated circuit (IC) chip is directly mounted on and electrically connected to its final circuit substrate instead of undergoing traditional assembly and packaging. The elimination of conventional device packaging in DCA both simplifies the manufacturing process and reduces the space that the IC chip occupies on the final circuit substrate. It also improves performance as a result of the shorter interconnection pathways between the IC chip and the circuit substrate.
Flip chips are frequently utilized in DCA applications. A flip chip comprises an IC chip with a multiplicity of solder bumps attached to the chip's bonding pads. During mounting, these solder bumps are directly attached to the circuit substrate. Once attached, the solder bumps serve several functions. Electrically, the solder bumps act to convey signals between the IC chip and circuit substrate. In addition, the solder bumps provide a thermal pathway to carry heat away from the IC chip. The solder bumps also serve to strengthen the mechanical attachment of the IC chip to the circuit substrate. Finally, the solder bumps act to form a space between the IC chip and the circuit substrate, preventing electrical contact between these elements. In the final stages of assembly, this space is usually filled with an electrically nonconductive “underfill.” The underfill protects the IC chip from moisture or other environmental hazards, provides additional mechanical strength to the assembly, and acts to reduce the strains in the solder bumps that result from thermal expansion differences between the IC chip and the circuit substrate.
In flip-chip-on-flex (FCoF) applications, the circuit substrate is substantially flexible, while in flip-chip-on-board (FCoB) applications, the circuit substrate is substantially rigid. In either case, the surface of the circuit substrate nearest the flip chip typically comprises a plurality of conductive traces, many of which are attached to the solder bumps of the flip chip. These conductive traces are usually supported by an organic material and act to conduct electrical signals between the flip chip and other electronic devices. In those applications where the circuit substrate is flexible, the surface of the circuit substrate opposed to the flip chip is often attached to a metallic stiffener. The stiffener prevents the circuit substrate from flexing in the region where it is attached to the flip chip. Moreover, in some applications, the stiffener is attached to a support structure that provides additional heat sinking capability above and beyond that of the stiffener.
In applications where the stiffener is attached to a support structure, increasing the rate of heat flow to the stiffener acts to increase the overall rate of heat dissipation from the flip chip. In a typical assembly comprising a flip chip, circuit substrate and stiffener, it is the solder bumps, underfill and circuit substrate that serve to transport the heat generated by the IC chip to the stiffener. Due to the low thermal conductivity of the underfill material, the primary heat dissipation path from the IC chip to the stiffener is through the solder bumps and the circuit substrate. In many flip chip designs however, solder bumps are only placed along the periphery of the IC chip. As a result, this primary heat path is limited by the cumulative cross sectional area of the solder bumps and the low thermal conductivity of the non-electrically conductive materials comprising the circuit substrate. Consequently, these thermal pathways are often inadequate to obtain a desired heat flow. When heat flow from the IC chip to the stiffener is inadequate, the IC chip's functionality and/or speed must be limited so that the IC chip does not exceed a temperature at which its lifetime is adversely affected.
Attempts to increase heat flow from the flip chip to the support structure in FCoF and FCoB applications include increasing the density of the conductive traces on the circuit substrate, increasing the thickness of the conductive traces, using higher thermal conductivity materials for the solder bumps, increasing the number of solder bumps and using higher thermal conductivity underfills. Unfortunately, these attempts typically result in only small improvements to the heat flow from the flip chip. Alternatively, an additional metal heat sink may be attached directly to the non-electrically active surface of the flip chip (i.e., the backside of the IC chip). While effective at increasing the heat flow from the flip chip, this solution results in added cost and may not be possible due to space constraints.
As a result, there is a need for improved flip chip and circuit substrate designs for use in FCoF and FCoB applications that enhance heat dissipation from the flip chip when compared to conventional designs.